The use of 4-arylazopyrazolone masking couplers is known in the art. See, for example, U.S. Pat. Nos. 2,455,170; 2,428,034; 2,808,329; 2,434,272; 2,704,711; 2,688,539; 3,796,574; 3,476,560; 4,427,763; EP 213,490; and U.S. Pat. No. 4,777,123 as well as the materials referenced in Research Disclosure December 1989, Section VII, Part G, Published by Kenneth Mason Publications, Ltd., Dudley Annex, 12A North Street, Emworth, Hampshire PO10 7DQ, England. These compounds have proven useful since they are yellow colored in nonexposed areas and magenta colored in exposed areas. Thus, while the magenta dye formed in a color negative photographic process has a small but significant unwanted absorption in the blue range, this may be balanced somewhat by the relative loss of blue absorption due to conversion of the mask color from yellow to magenta in the exposed areas. Then, an adjustment can be made to the spectral content of the light used to produce the positive from the negative to effectively cancel out the unwanted blue absorption which has become essentially constant across both the exposed and unexposed areas of the negative.
It is known that certain substituents on the 4-arylazo group are useful. Included are alkoxy, hydroxy, and carbonamido groups, usually in the para position to the azo function. These functions have not been generally associated with any particular feature in the pyrazolone nucleus. It has been customary to include such substituents as will permit or indeed improve the propensity of the decoupled arylazo residue to be washed out of the film during processing.
While 4-arylazopyrazolone masking couplers have been employed as a means of offsetting the unwanted blue absorption of conventional magenta couplers, this means for improving the color rendition has created several other problems in the photographic material. First, the presence of these masking couplers results in increased fogging (silver development at no exposure) of the photographic emulsion. This is thought to be due to reaction during development between the oxidized developer and the masking coupler which results in the formation of an aryldinitrogen species. This species can then act as a powerful reducing agent for silver emulsions. The result is undesired silver development which manifests itself as fog. Thus, the Dmin (minimum density) of the photographic material is undesirably increased due to the presence of this class of masking coupler. This deficiency is amplified even further in the case of processing using extended development times ("push" processing) where due to the extended processing time the extent of fogging is increased even further.
An added deficiency is the loss in Dmax with time during raw stock keeping. Apparently a dinitrogen species is formed which attacks the image coupler and results in a loss in the capacity of the image coupler to be converted to image dye.
A still further deficiency with the 4-arylazopyrazolones is their poor masking efficiency. It is desirable to maximize the differentiation between the yellow of the dye before processing and the magenta of the dye formed upon reaction with oxidized developer. In order to effectively remove blue unwanted density as seen by a sensing element (a printer monitor or photographic paper), the yellow form of the coupler must have as high absorbance as possible in the same regions of blue sensitivity as the receiving element (typically 480 nm for color paper) while minimizing the amount of green absorbance as seen by these same sensors. Conversely, the magenta dye formed from the coupler after reaction with oxidized developer should have as low as possible absorbance in the blue region of receiver sensitivity and as high as possible in the green region. In particular, it is desirable to maximize the difference in absorbance at 480 nm between the yellow colored phenylazopyrazolone and its magenta colored dye formed by the reaction with oxidized coupler. Masking efficiency is also impacted by the reactivity of the coupler. While the addition of electron withdrawing substituents to the parent group might be expected to reduce the activity of the coupler, this effect was not found to be significant in this invention.
Higher masking efficiency couplers allow for reduced laydown of the masking coupler and/or for better color reproduction since the masking coupler has improved absorbance in the desire range and reduced absorbance in the undesired range.
It is also desirable to employ such couplers which exhibit mass efficiency , i.e. achieve the desired photographic properties with use of lower molecular weight materials than heretofore employed, thereby reducing manufacturing, disposal, and recycling costs.
In EP 213,490 there are shown masking couplers having two alkoxy substituents on the arylazo ring. While a parent group having a sigma(para) sum exceeding 1.4 is shown (II-16), such a parent is not shown in combination with an arylazo portion meeting the requirements of this invention and the advantages of the invention are not realized. In a similar manner, Japanese Published Application 58/172,647 shows masking couplers ( e.g. CM-21 to CM-25 ) but the parent does not have the required sigma(para) value and/or the arylazo ring substituents do not meet the requirements specified in this invention and the desired results are not obtained.
It is therefore a problem to provide a masking coupler capable of masking the unwanted blue absorption of a magenta dye which masks more efficiently than the currently employed 4-arylazopyrazolone masking couplers and for which embodiments do not substantially increase fogging or reduce maximum density.